clone

Clones of a stream access the same data, and are positioned at the same
point as the stream they were cloned from.

Expert: Subclasses must ensure that clones may be positioned at
different points in the input from each other and from the stream they
were cloned from.

Warning: Lucene never closes cloned
IndexInputs, it will only do this on the original one.
The original instance must take care that cloned instances throw
AlreadyClosedException when the original one is closed.